On the Symfony blog today there's a new post from Fabien Potencier talking about the upgrade path to keep your Symfony projects up to date. He points out that in the pre-Composer days it was more difficult, but thanks to this handy tool, it's a much easier task.

In the old Symfony 2.0.x days, at a time when Composer did not exist, upgrading a project from one minor version of Symfony to the next one was not that easy. [...] Fast forward to the Composer world. Composer is used by Symfony since version 2.1, but as we were the very first adopter, and because Composer and the Symfony ecosystem were not stable yet, it was still awkward. [...] Fast forward to Symfony 2.3.
As of Symfony 2.3, the minimum stability level has been raised to stable.

This stability means that all of the libraries and tools that Symfony 2.3 depends on are in their "stable" state as well. Included in the post are the commands to update your Symfony install and what it looks like when the update from 2.3.0 to 2.3.1 happens. He finishes off the post with a few recommendations about how to use Composer to keep things stable (use "stable", define version constraints and run the update without fear of breakage).